MS Word treats each letter input as … (see attached image):
The one in green box was typed using English keyboard that comes with Windows while the one in the red box was typed with Keyman keyboard.
Keyman has a fairly large user base. Enough to get the attention of large corporations (examplehere). If I were to want to investigate this issue further, I would install some of the most popular keyboards on Keyman and see if I can reproduce the issue. Then I would make a bug report with Microsoft. Worth a shot!
I suggest that whatever keyboard you choose should make sure it doesn’t use any of the shortcut keys that Microsoft Word uses (CTRL+C), (ALT+A), etc, etc. Otherwise they might get derailed by that issue.